Existing tanker-based floating production systems evolved from tanker mooring terminals. After initial successes with these simple systems, more sophisticated types were developed to broaden the operational capabilities. For the purpose of putting the present invention into perspective, there are two fundamentally different types of systems. The difference is in the tanker mooring method and in the riser which connects the wellheads on the seabed to the tanker.
One type of floating production mooring system consists of a buoy anchored to the seabed by a conventional catenary mooring spread. The tanker is attached to the buoy by a hauser and is free to swing around the buoy as the sea conditions change. The risers with this system are flexible hoses.
The other type of floating production mooring uses a single anchor leg or tower instead of a catenary moor, and a rigid link or yoke connecting the tanker to the tower. Again the tanker is free to weathervane around the tower. In this case the tower acts as the riser as well as the mooring device. The yoke has hinges which allow the tanker to move freely, without pulling or compressing the tower.
The present invention relates more to the single anchor leg, but a knowledge of the differences in the loading of the mooring system will help in the understanding of the invention. One difference between catenary moor and the single tower is that a catenary anchor line only acts in one direction, so many lines are required for multidirectional load capability. But the main difference is in the anchoring at the seabed. The tower, being rigid, puts a high vertical load into the seabed whereas the catenary moor relies on heavy chain weight and puts a horizontal load into the seabed.
But at the surface, the principle is the same for both systems. The restraining force is provided by the horizontal component of the tension in the anchor line or tower.
Dealing now only with the tower, the tension is provided by buoyancy, either in the top of the tower or in the yoke connection to the tanker this is the basis of the "SALS" system.
The tower system is designed to suit the water depth and sea conditions of a specific site. Thus, to move the tower to a different location would require modifications to suit the new water depth. The system is also permanent in that the release of the tanker requires a significant decommissioning operation. Similarly, the buoyant yoke assembly, although attached to the tanker by hinges, becomes a permanent part of the tanker, making it difficult for the tanker to move location in bad sea conditions. When considering deep water, the tower system has operational limitations. Because the system relies on the tower being at an angle to provide tanker restraint, i.e. a horizontal component of tension, the top of the tower swings downward as the angle of the tower increases. This vertical displacement is proportional to water depth. In deep water the yoke either requires greater movement or the buoyancy force must be increased to reduce the angular requirements of the tower. Either way, the whole system becomes larger, reducing its practical and economic viability.
Catenary anchor systems. although slightly less permanent than tower/yoke systems, have similar limitations. Movements and chain sizes become impractical in severe sea conditions and deep water.
The yoke is common to most of the larger facilities. It is coupled to the ship with hinges, on its beam girth line. The yoke is necessarily large for the following reasons:
Its length provides heave and pitch freedom and its width must be such to allow direct mounting to the bow or stern of the ship at its girth line;
It is heavy so as to be structually capable of handling very large tensile, compressive, and torsional loads due to mooring and wave action.
In all cases, the yoke only has freedom to hinge up and down. Whenever the ship rolls, the structure must follow the ship, hence loading the hinge pins and twisting the relatively long yoke about the riser/tower/buoy connection. This is a serious load problem. Sway also "drags" the entire yoke to the side further complicating the force combination at the hinges.
Suffice to say that the yokes are extremely robust and correspondingly heavy. Even the smallest ones, used in quite moderate sea conditions, weigh 500-600 tons. The best known unit, TAZERKA, has a yoke weight of over 2000 tons.
Buoy systems "disappear" on crossing the 500 ft. depth boundary. Towers with associated yokes also lose favour at 600 ft. depth. The reasons are that the deeper water means more chain length for the buoy: it gets bigger, catches more wave loading and ruins the yoke-buoy connection. For towers, towing it out horizontally and uprighting it is critical: too much bad treatment and it bends.
For the "SALM" systems, which introduce an articulation at the centre of the tower, there is an improvement. However, a system has not yet been installed in deep water.
The "SALS" system tends to stand out on its own, but again, it is presently bounded by the "tower" weakness which also limits the system to a specific, shallow water site.
One thing common to all these known yoke systems, is that the riser/swivel/manifold unit is remote. That means access problems to the riser itself. All these systems impose limitations on themselves, especially their access features, by answering only the strictly functional, mooring, problems. To say nothing of deployment.
The features of the present invention attempt to address as many of the functional and operational aspects as possible, most benefits being realized from the unique motion compensation arrangement.
The objective of the present invention is to overcome the above mentioned limitations of the art and to provide a tanker-based floating production system that is very mobile and relatively insensitive to water depth, featuring an inexpensive, passive motion compensation system.
This objective is achieved in part by having a riser that is made up from 50-ft. sections and deployed from the production tanker. The riser is lowered from the tanker as it is made up, locked to a riser base on the seabed, and tensioned by an internal float motion compensator on the tanker. The tanker is then allowed to move away from its original position under the action of wind, waves and current until the riser is at a sufficient angle to stop further tanker movement. As in the tower and yoke systems, the horizontal component of the riser tension provides the restraining force on the tanker.
Flotation provides substantial forces, which are considered "free". Hydraulics will do the same, but with unwanted complexity and expense.
Floats in the sea beside a ship pick up waveinduced forces. If they are attached to push rods, levers, cage structures or other devices, they invariably have to move around in the water, inducing high loads in the linkages, etc. Basically, having floats attached to the ship, external to the hull, is not an intelligent way of finding free forces for mooring. Whenever the ship rolls, for example, so must the float, often at its worst extension. This causes problems of friction, roll amplification, unwanted structural loads, etc.
The SALS system is a prime example of a float external to the ship, which must be held in a massive structure just to survive its demanding environment.
All the buoy mooring systems have the same problem, as mentioned previously. As depths and sea states get more demanding, the buoyancy must be increased. However, a definite limit is reached; if this limit is ignored, the only way to make the system work is to make structures, floats and bearings very large, clumsy and expensive.
By putting float devices within the ship in accordance with the present invention some clear advantages are observed:
not influenced by wave induced forces, or splash zone pounding;
floats roll, pitch, yaw, sway and surge with the ship;
it is a controlled environment with good access;
operators can observe and monitor float behaviour, conditions;
buoyancy can be controlled directly using compressed air to de-ballast the floats;
the S.G. of the surrounding medium can be altered to derive optimum buoyancy, viscosity;
travel of the float or heave is a fraction of the ship's heave;
float accelerations and velocities (heave) are also a fraction of the ship's values;
float shapes can be more innovative due to the better defined operating environment;
the float is totally self-contained within the ship and needs no deployment steps whatsoever; and
the float can be used to provide base forces during riser deployment.